A superconducting mixed phase yttrium-barium-copper-oxygen compound system having a superconducting transition temperature, T.sub.c, was disclosed in Physical Review Letters. Vol. 58, 908 (Mar. 2, 1987), which has become known in the art as the 123 material short form for the formula YBa.sub. Cu.sub.3 O.sub.7-y, wherein y is in the range 0.1 to 0.5. Depending on the precise conditions utilized to make the 123 compound and upon other process condition factors, the 123 compounds exhibit a T.sub.c in the range of about 91.degree. to 93.degree. K. The 123 material, however, can lose oxygen from its crystal structure, particularly when subjected to heat and/or pressure. The loss of oxygen may degrade not only the superconducting properties, such as the temperature at which the material is superconductive but also, the charge carrying ability of the material, both under the superconducting conditions and under non-superconducting (metallic) conditions.
Other disadvantages of the 123 materials are that they are highly anisotropic, are poor conductors in the normal (non-superconducting) state, and contain undesirable twin planes in their lattices (a characteristic which contributes to anisotropy). Poor normal state conductivity is a limitation for use as a wire or magnet. Anisotropy may reduce critical current density, therefore very oriented, isotropic materials would be preferable, which optimally should be single crystals. The crystallographic defect of twinning causes weak links in the lattice and reduces critical current density. The 123 materials also exhibit orthorhombic distortion which is not ideal for epitaxial growth of thin films.
Furthermore, loss of oxygen from the 123 materials occurs from the CuO chains, and excessive loss degrades the superconductivity. The lability of oxygen in the 123 materials is even observed in relatively unsevere conditions (400.degree. C. in argon or 600.degree. C. in oxygen). High vacuum conditions would enhance oxygen loss. Oxygen deficiency would be most pronounced at the surface, which would thus be problematic for use in tunnel junction based devices.
Even when prepared in single crystal form, T.sub.c 's for 123 materials have not been observed to exceed about 92.degree. K. Materials having higher T.sub.c 's would be preferable.
It is therefore an object of the present invention to provide novel oxide materials in which the oxygen atoms are locked and therefore providing more stability than the 123 compounds.
It is a further object of the present invention to provide novel superconducting compositions which have an entirely different crystal structure from the 123 materials.
It is a further object of the present invention to provide novel superconducting compositions in which single atomic layers of the 123 material appear as defects in an ordered defect structure.
It is yet another object of the present invention to provide novel mixtures of the aforementioned novel superconducting compositions with the 123 material.
These and other objects of the invention will be apparent from the following description and from the practice of the invention.